278 



ferent formations of rock cover the gneiss. We 

 shall first give their description, without group- 

 ing them according to systematic ideas. 



On the South of the Cerro de Chacao, be- 

 tween the ravine of Tucutunemo and Piedras 

 Negras, the gneiss is concealed beneath a for- 

 mation of serpentine, of which the composition 

 varies in the different superimposed strata. 

 Sometimes it is very pure, very homogeneous, of 

 a dusky olive green, and of a conchoidal frac- 

 ture passing to even : sometimes it is veined, 

 mixed with bluish steatite, of an unequal frac- 

 ture, and containing spangles of mica. In both 

 these states I could not discover in it either 

 garnets, hornblende, or diallage. Advancing 

 farther toward the South, and we always passed 

 over this ground in that direction, the green of 

 the serpentine grows deeper, and feld-spar and 

 horneblende are recognized in it : it is difficult 

 to determine, whether it pass into diabasis 

 (gruenstein), or alternate with it. There is no 

 doubt however, of it's containing veins of copper 

 ore *. At the foot of this mountain two fine 

 springs gush out from the serpentine. Near the 



* One of these veins, on which two shafts have been sunk , 

 I was directed hor. and dipped 80° East. The strata of the 

 serpentine, where it is stratified with some regularity, run 

 hor. 8, and dip almost perpendicularly. I found malachite 

 disseminated in this serpentine, where it passes into gruen^ 

 stein. 



